One of the most remarkable aspects of the marijuana movement has been the lack of an organized challenge from anti-drug crusaders. After decades of militarized DEA raids, skyrocketing prison populations, and moralistic warnings about hippies and gateway drugs, marijuana-reform advocates have come flooding back from political exile basically unopposed, ushering in a new era of taxed-and-regulated weed sales in Colorado and Washington and pushing through measures to legalize medical marijuana in 20 other states. As the Drug War’s Iron Curtain has fallen, prohibition hardliners have so far been silent, putting up surprisingly feeble resistance to the generational shift in public opinions about pot.

But in Florida, a donation from one of the Republican Party’s most powerful megadonors has signaled that legalization opponents might be ready to pick up the fight. According to campaign finance reports filed last week, gambling mogul Sheldon Adelson has spent $2.5 million to defeat a constitutional amendment to legalize medical marijuana in Florida. The donation accounts for nearly all of the $2.7 million raised by the Drug Free Florida Committee, a marijuana opposition group started by GOP donor Mel Sembler, a Bush-era Drug Warrior who used to run a chain of teen rehab black sites best known for raping, kidnapping, and torturing clients.

Sarah Bascom, a spokesperson for the Drug Free Florida Committee, said she could not say why Adelson had made the donation, but that “we are grateful for his support.” While Bascom would not comment on how the organization plans to use the funds, she did say that the group believes the amendment is vulnerable and plans to keep raising money “to educate people on what the amendment really is, and what it is not.”

Political strategists on both sides of the marijuana issue say they are perplexed by Adelson’s donation, which appears to be his first contribution to an anti-marijuana group. While he is the GOP’s most generous donor—he spent an estimated $150 million on Republican campaigns in 2012 alone—his political agenda has been singularly focused on protecting his gambling interests and pushing a hardline, pro-Israel foreign policy. (Sembler is also an octogenarian Israel hawk and the two men are apparently friends.) But Adelson, the CEO of Las Vegas Sands Corp., and the eighth-richest man alive, isn’t known for being a prude or a prohibitionist. In fact, a medical research foundation founded by Adelson and his wife in Tel Aviv recently published a study that found marijuana could potentially ease symptoms of MS and other inflammatory diseases.

By comparison, prohibitionists have raised almost no money. In Washington, for example, pro-pot groups spent more than $6 million to back the 2012 legalization initiative; opponents raised just $15,995. In Colorado, anti-drug groups backed by Sembler managed to raise $577,410, but were still outspent 4-to-1 by legalization activists. “Generally, there has not been a whole lot of moneyed opposition to these initiatives,” said Ben Pollara, campaign manager at United For Care, the Florida drug reform group leading the medical marijuana amendment campaign. “There are not a lot of business or ideological interests that are against this issue. It’s just not controversial for most people.”

In Florida, though, the opposition is more muscular and organized, bringing together the dour cross-section of concerned parents, law enforcement officers, and Drug War neocons that united against marijuana in the last century, but has been mostly irrelevant in recent state ballot campaigns. Marijuana activists concede that these anti-pot campaigns could be problematic in the Amendment 2 fight, forcing groups like United for Change to spend more money on TV ads and increase fundraising efforts in the lead up to this fall’s vote.

“When they spend millions of dollars spreading misinformation about marijuana and exaggerating its harms, it’s certainly possible that it could have a negative effect,” said Mason Tvert, director of communications for the Marijuana Policy Project. “But most Americans have been hearing this misinformation for a long time, and most Americans are starting to see through it. Hopefully they won’t be fooled by new efforts to scare people.”

Recent polls have shown overwhelming support for medical marijuana legalization across Florida, and across voter demographics. A survey released by Public Policy Polling last week found that about 66 percent of voters would vote for Amendment 2. Even Republican voters were split on the issue, with 44 percent saying they would vote for the legalization. “The numbers show this cruising to pass in the fall,” said Rick Wilson, a Florida-based Republican strategist. “It would take an awful lot more than $2 million to change that.”

Pointing out that Florida has some of the most expensive media markets in the country, Wilson added that the Adelson donation amounts to “seed money, at best—it’s not going to be able to actually change the way people think about this issue.”

But Adelson’s foray into the marijuana wars is also a sign that the legalization movement has transformed into a professional Campaign Issue, one that political strategists can use to drive voter turnout, partisan attacks, and Big Money donations. The $2.5 million donation has turned the Amendment 2 campaign into a proxy battle in Florida’s gubernatorial election, pitting Democratic candidate Charlie Crist and his pro-pot supporters against Republican lawmakers who claim the issue is just a ploy to turn out liberal voters in November. And unlike in Colorado and Washington, where Democratic governors largely stayed out of legalization campaigns, Florida politicians have been forced to take sides, laying out new political fault lines that could reverberate in other states.

“This out-of-state financial assistance from a long-time Republican political player is further evidence that leaders of both political party persuasions understand the stakes involved,” said Paul Armentano, deputy director for NORML, which supports marijuana reforms. “Florida is a bell-weather state and passage of marijuana law reform, for the first time, in the southeastern United States would likely have sweeping national ramifications.”